Method of making composite metal strips



April 6, 1943. J, A, rrz' 2,315,657

METHOD OF MAKING COMPOSITE METAL STRIPS Filed July 26, 1940 2 Sheets-Sheet l INVENTOR John A..Hitz

April 6, 1943.

J. A. RITZ 2,315,657

METHOD OF MAKING COMPOSITE METAL STRIPS Filed July 26, 1940 2 Sheets-Sheet 2 FIG. 2

FIG. 4

INVENTOR M Ma AflM.

Patented Apr. 6, 1943 METHOD OF MAKING COMPOSITE METAL STRIPS John A. Ritz, Youngstown, Ohio, assignor, by

mesne assignments, to Metal Car-hides Corporation. Youngstown,

Ohio, a corporation of Ohio Application July 26, 1940, Serial No. 347,671

-'3 Claims.

This invention relates generally to coated bodies, for example tin plate, made by cold rolling a steel body in contact with rolls made of hard metal carbide composition. It relates also to the method of producing such coated bodies and particularly to steel sheet or strip coated with tin or other protective metals, the steel body having such a smooth surface that less tin is required to produce a coated body of the same freedom from porosity.

Fig. 1 is a microphotograph of a low carbon steel made by cold rolling the steel strip using the ordinary steel rolls generally employed for such process. The magnification is 200 diameters.

Fig. 2 is a chart made by a Brush cardiograph, which measures the smoothness of the surface of an article.

Fig. 3 is a microphotograph, the magnification being 200 diameters, of the same steel shown in Fig. 1 cold rolled in the same manner as the steel of Fig. 1, except that tungsten carbide rolls were used in place of steel rolls; and

Fig. 4 is a chart similar to Fig. 2 recording the surface characteristics of the steel rolled with the carbide rolls.

In the production of tin plate, hot rolled strip is cleaned by pickling, then cold rolled to mas terially reduce its thickness, is then normalized or annealed in order to relievev strains imparted by the cold rolling and is then given a temper or skin rolling in order to produce the desired final thickness and impart the desired physical characteristics to the strip. The strip is then coated with tin, either by the molten tin or electrolytic process. in carrying out the cold rolling operations, steel rolls have been employed. When strip has been prepared by the above method for coating with tin, it has been the custom to use about 1.50 or more poundsof tin per base box in the hot dip process in order to insure that the tin plate will meet the required specifications as to freedom from porosity and to insure suitable soldering ability.

The amount of tin, of course, varies with the degree of porosity allowable in the fianl product. The outstanding problem has been present for many years of producing the desired freedom from porosity without using excessive amounts of tin. Various methods of preparing the strip for tinning and various methods have been employed in the actual tinning step itself, but up to the present time, so far as I am aware, it has been necessary in the hot dip process to use at least 1.3 pounds of tin per base box, in order to insure that when the tin plate is tested by the standard hydrogen evolution method it will show a value of 37 hours.

T have found that by cold rolling the strip using hard metal carbide rolls in place of the prior known steel rolls 1 can impart such a smooth surface to the strip that the amount of tin orother protective metal may be greatly reduced and still meet the requirements as to freedom from porosity. Conversely, if the same amount of tin is used, the porosity of the strip is greatly reduced. The hardimetal carbide composition which I prefer to use in making the rolls is one containing tungsten carbide and a binder such as nickel or cobalt. A method of making such rolls is described and claimed in DeBats and Ritz Patent 2,167,544, granted July 25, 1939. So far as the present invention is concerned, any of the known hard metal compositions may be employed for making the rolls. Among the most commonly used hard metal carbides are the carbides of tungsten, tantalum, titanium, molybdenum, chromium and vanadium, these carbides being bonded by iron, nickel, cobalt or other metals or alloys. The hard metal carbide compositions are made by a cementing or sintering operation.

As illustrative of the improvement produced by using hard metal carbide rolls in place of steel rolls in cold rolling strip preparatory to coating with tin by the molten tin process, the following example is given. Each of three coils of 18 wide hot rolled strip which were .105 in thickness was cut into two portions and one portion of each coil was rolled with steel rolls, whereas the other portion of each coil was rolled with tungsten carbide rolls. The composition of the steel was:

Carbon .09 Manganese .36 Phosphorus .009 Sulphur .032

A part of one of the coils was cold rolled from .105" to .011" in 16 passes using steel rolls. It was then normalized at 1750 is. at 29 feet per minute and was then cold rolled from .011 to .010" using steel rolls.

The other part of the same coil was rolled, normalized and otherwise treated under substantially identical conditions, except that tungsten carbide rolls were used in the cold rolling steps. This portion of the coil was cold rolled from .105 to .011" in 21 passes using tungsten carbide rolls. It was then normalized at 1750 F. at 29 feet per minute. It was then cold rolled from .011" to .010" using tungsten carbide rolls.

The microphotograph of Fig. 1 illustrates the surface obtained by using the steel rolls, Whereas the microphotograph of Fig. 3 illustrates the surface obtained by using tungsten carbide rolls. It will be noted by comparison that although all of the conditions were the same except as to the kind of rolls used, the tungsten carbide-rolled strip is much smoother and freer from pits than is the steel-rolled strip. The charts obtained by the use of the Brush cardiograph clearly lllus-. trate the difference in surface characteristics of the two materials. This instrument has a needle which travels over the surface the characteristics of which are to be measured and the characteristics are recorded on a chart. It will be seen that the hills and valleys shown in Fig. 2 are of much greater dimensions than those shown in Fig. 4. The roughness of the steel rolled material is many times that of the tungsten carbide-rolled materials.

The entire two lots of coils, that is the portions of the coils which has been rolled with the steel rolls and the portions of the coils which had been rolled with the tungsten carbide rolls, were then coated with tin by the ordinary hot dip process using molten tin. The material was put through the tin pot and the steel rolled plate took from 1.3 to 1.6 pounds of tin per base box.

base box, was freer from porosity than the steel' rolled material containing from 1.3 to 1.6 pounds of tin per base box. It is believed that the quantity of tin may be materially reduced even below the .86 pound per base box above referred to down to about .5 pound of tin per base box and still meet the porosity test requirements.

Other tests have been made on'carbide rolled strip which has been tin plated with larger amounts of tin than the .86 to 1.00 pound per base box above referred to. In carrying out these other tests, the steel strip was of the same general analysis previously referred to and was cold rolled using tungsten carbide rolls for both the rough cold rolling and the temper cold rolling operations. The strips were normalized between the rough and temper cold rolling steps as previously described and after the temper cold rolling they were coated with tin in the amounts given in the following table and tested for porosity by the standard hydrogen evolution method, Table shows the correla tion between the coating weight of the tin and the number of hours as obtained by the hydrogen evolution test.

Table Hydrogen evolution, hours Coating wt. of tin, lbs.

Sample per base box It will be noted from the table that sample B-l .even pass a 37 hour nated.

hydrogen evolution test I The use of hard metal carbide composition rolls in place of steel rolls results in other improvements than a reduction in the amount of tin or other coating metal which is required to be used. The tin plate made from carbide rolled strip has what may be termed a blue" color, while tin plate made from strip rolled with steel rolls has a gray" color. Tin plate made from carbide rolled strip has-an appearance similar to charcoal plate having a coating of about three pounds of tin per base box, even though the carbide rolled strip has only .86 pound of tin per base box. Both of these products have oil lines, which is one of the characteristics of charcoal plate. Thus the appearance of tin plate made from carbide rolled strip but containing relatively small amounts of tin is comparable to the moreheavily coated charcoal plate made by coating strip rolled with steel rolls.

The steel strip rolled with carbide rolls has 30 to 40% greater ductility, as shown by the Schopper bend test than does a strip made by rolling with, steel rolls, each of the strips having approximately the same Schopper yield points. This increase in ductility is important in many applications of the tin plate. In the formation of tin cans, for example, it enables the body blanks to be bent so as to receive the ends of the can and yet the body of the can will have suflicient strength to withstand deformation in use. The improvement in ductility for the same yield point is another indication of the very smooth character of the surface produced by the use of carbide rolls. The surface produced with the carbide rolls is-so smooth as to be substantially comparable to the metallographic surface produced by hand polishing specimens for photomicrograph work. The defects in the surface are so extremely minute that they,do not provide incipient faults which when of greater magnitude, as in the steel-rolled plate, afford the basis for the start of a defect such as a crack and the consequent lessened ductility.

Another advantage of the carbide rolled strip is that the amount of white pickling after normalizing and prior to the temper or skin cold rolling step may be materially reduced or elimi- Ordinarily in carrying out this white pickling operation a 2/z% acid solution is used, the duration of the pickle being from 4 t0 5 minutes. However, when the strip has been produced by the use of carbide rolls, satisfactory results have been obtained using an acid strength of 1%, the duration of the pickle being three minutes. It is believed that this white pickle could be reduced in half and still obtain satisfactory results. 3

In order to secure the best results, it is necessary to do the rough cold rolling prior to heat treatment and then the temper rolling after the heat treatment with carbide rolls. Better results than ordinarily secured are obtained if the breaking down is done on steel rolls and the temper rolling with carbide rolls but these re sults are not nearly as good as if the carbide rolls are used for the two rollings.

The invention has been described particularly in connection with the production of tin platei. e., a product in which the coating metal is tin. The invention, however, is not restricted to the use of tin as a coating metal, since the preparation of a very smooth surface by the use of rolls of hard metal carbide composition will give improved results, irrespective of the particular coating metal which is applied to the base metal. For example, similar improved results would be obtained by carrying out the present invention using coatings of nickel, silver, chromium or other suitable protective metals. Furthermore. the advantages of the very smooth surface obtained by the use of carbide rolls will apply whether the coating metal is applied in a molten condition or by electrodeposition. The invention, therefore. is not limited to molten applied tin or other coating metal but covers electrodeposited metal as well. Also, the invention is not limited to the use of steel or other ferrous metal as the base to which the coating metal is applied. It covers the use of any other suitable base for receiving the protective coating.

Although I have described the present preferred embodiment and manner of carrying out the process, it will be understood that the invention may be otherwise embodied or practiced Within the scope of the following claims.

I claim:

1. The method of producing steel strip or the like suitable for tin plate, which comprises cold rolling a steel strip by contact with rolls made of hard metal carbide composition to materially reduce its thickness, annealing the strip and temper rolling it by contact with rolls made of hard metal carbide composition to impart a very smooth finish thereto.

2. The method of producing tin plate, which comprises cold rolling a steel body by contact with rolls made of hard metal carbide composition to materially reduce its thickness, annealing the strip and temper rolling it by contact with rolls made of hard metal carbide composition to impart a very smooth finish thereto, and thereafter applying a molten tin coating to the body.

3. The method of producing a metal body having a coating of protective metal thereon, which comprises cold rolling a metal body by contact with rolls made of hard metal carbide composition to materially reduce its thickness, annealing the metal body and temper rolling it by contact with rolls made of hard metal carbide composition to impart a very smooth finish tl: ereto, and thereafter coating the body with protective metal.

JOHN A. RITZ. 

